III-Nitride light emitting devices are based on semiconducting alloys of nitrogen with elements from group III of the periodic table. Examples of such III-Nitride devices include InxAlyGa1-x-yN light emitting diodes (LEDs) and laser diodes (LDs).
The active regions of InxAlyGa1-x-yN LEDs and LDs typically include one or more InxAlyGa1-x-yN quantum well and barrier layers. These layers typically have alloy compositions which differ from each other and differ from the surrounding layers in the device. As a consequence of these composition differences, the layers in the active region of an InxAlyGa1-x-yN light emitting device are typically biaxially strained. It should be noted that in the notation InxAlyGa1-x-yN, 0≦x≦1, 0≦y≦1, and x+y≦1.
InxAlyGa1-x-yN crystals such as those from which InxAlyGa1-x-yN light emitting devices are formed typically adopt either the wurtzite or the zinc blende crystal structure. Both of these crystal structures are piezoelectric. That is, both develop an internal electric field when stressed. In addition, the low symmetry of the wurtzite crystal structure produces a spontaneous polarization. As a result of their biaxial strain and of the piezoelectric nature of InxAlyGa1-x-yN, and as a result of the spontaneous polarization (when present), the quantum well layers and barrier layers in an InxAlyGa1-x-yN light emitting device typically experience strong internal electric fields even when the device is unbiased.
For example, FIG. 1 shows a schematic band structure diagram for a portion of an unbiased prior art InxAlyGa1-x-yN LED active region including GaN barrier layer 2, InxGa1-xN quantum well layer 4, and GaN barrier layer 6. The two horizontal axes in FIG. 1 represent position in the active region in a direction perpendicular to the layers. The interfaces between the layers are indicated by dashed lines. The lower vertical axis represents the energy of the conduction band edge 8 and of the valence band edge 10 in the various layers. The upper vertical axis represents the concentration of indium in the alloys from which the various layers are formed. Layers 2, 4, and 6 are of wurtzite crystal structure with the c-axis of the crystal substantially perpendicular to the layers and directed from layer 2 toward layer 6. In the prior art active region, the mole fraction of indium is constant across the width of quantum well layer 4.
In the absence of a spontaneous polarization, piezoelectric fields, and an externally applied bias, the conduction band edge 8 and valence band edge 10 would be nominally flat within each layer. In the band structure depicted in FIG. 1, however, piezoelectric fields have tilted the band edges. This tilting adversely affects the performance of a light emitting device including the illustrated active region. As a result of this tilting, for example, the electron wave function 12 and the hole wave function 14 are concentrated on opposite sides of InxGa1-xN quantum well 4. The spatial overlap of these wave functions is therefore reduced by the piezoelectric field, leading to a decrease in the probabilities of spontaneous and stimulated emission of light from the active region and an increase in the probability that electrons and holes injected into the active region will relax nonradiatively or leak out of the active region. Hence, the piezoelectric field decreases the operating efficiency of InxAlyGa1-x-yN LEDs and the optical gain in InxAlyGa1-x-yN LDs. Consequently, the piezoelectric field makes high brightness InxAlyGa1-x-yN LEDs and low threshold InxAlyGa1-x-yN LDs difficult to achieve.
Another consequence of the piezoelectric field in InxAlyGa1-x-yN light emitting devices is a reduction of the emission energy. Charge injected during operation of the device partially screens the piezoelectric field, however, and results in an increase of the emission energy as the carrier density in the quantum well layer is increased. In high-indium-content quantum well layers this shift can result in drastic color changes with variation in injection current.
What is needed is an InxAlyGa1-x-yN light emitting device in which the problems associated with the internal piezoelectric field have been overcome.